1. Field of the Invention
The present invention relates generally to a shift control system for an automatic power transmission for an automotive vehicle. More specifically, the invention relates to a shift control system which has a capability of controlling an appropriate lag time from entry of a shifting command to an actual occurrence of a variation of a transmission speed ratio.
2. Description the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 57-144338 discloses one example of a shift control system for an automatic power transmission. The disclosed shift control system includes a 3-2 timing valve. The 3-2 timing valve controls drain speed of a working fluid in a third speed servo release chamber of a hydraulic servo device in a band brake device. In the known hydraulic circuit, a control pressure is supplied to the third speed servo release chamber and to a third speed clutch through a common hydraulic line. The 3-2 timing valve is disposed in the line connected to the third speed servo release chamber in a parallel relationship with a flow restriction orifice. The 3-2 timing valve establishes and blocks a by-pass line by-passing the orifice. With the shown construction, the 3-2 timing valve is held open to establish fluid communication through the by-pass line for rapidly draining pressure in the third servo release chamber, when vehicle speed is relatively low. On the other hand, at a relatively high vehicle speed, the 3-2 timing valve is maintained at a shut-off position for blocking fluid communication through the by-pass line. As a result, the pressure in the third speed servo release chamber is drained via the flow restriction orifice. By this, a lag time from entry of a shifting command to engagement of band brake can be provided.
However, timing adjustment to be performed by the conventional shift control system set forth above is not at all completely satisfactory. Namely, when the 2-3 shift valve is switched into the second speed position, pressure applied to the clutch is started to be drained. Simultaneously, the pressure in the third speed servo release chamber is started to be drained. Draining of the pressure in the third speed servo release chamber is started irrespectively of the valve position of the 3-2 timing valve. Therefore, draining is started at the identical timing, i.e. simultaneously with draining of pressure applied for the clutch, when the 3-2 timing valve is in a open position to establish fluid communication via the by-pass line and when the 3-2 timing valve is in a shut-off position to block fluid communication via the by-pass line. If the vehicle speed is relatively high to place the 3-2 timing valve at the shut-off position, the pressure in the third speed servo release chamber is drained via the flow restriction orifice for a lower drain speed. Therefore, the speed of lowering of the pressure in the third speed servo release chamber becomes lower than that of the clutch. As result, a difficulty is encountered to provide a sufficient lag time between releasing of clutch and engaging of the band brake. This further results in difficulty providing a sufficient period to maintain a line pressure at a transition neutral pressure. This degrades a shift feeling of the power transmission upon variation of the speed ratio.
In order to solve the aforementioned problem, Japanese Patent First Publication (Tokkai) Showa 63-96345 discloses an improvement of the shift control system, in which a one-way valve is provided for controlling fluid flow of the working fluid. In the disclosed system, the control pressure is directly supplied from one switching valve to one frictional element, and the control pressure from the one switching valve is supplied to the other frictional element via the one-way valve. The timing valve is provided in the line for the other frictional element. With such construction, if the pressure in the aforementioned one frictional element becomes higher than that in the other frictional element, the fluid pressure difference becomes small since the pressure drained from the one frictional element flows into the other frictional element via the one-way valve. This makes flexibility of adjustment of the timing more limited.